Power torque wrenches are used where many threaded fasteners are employed or when a threaded element must be employed with a limit on the amount of torque applied to it. Typical examples are connecting two large diameter pipes together with a pair of flanges wherein many nut-bolt combinations must be taken up and tightened to a predetermined torque. A power torque wrench saves hours of labor as compared with using a handoperated torque wrench and is especially useful where high torque must be employed.
A power torque wrench must have a small enough head to be used in the space available for its operation. For example, when taking up and tightening nut-bolt combinations between two flanges, the head of the torque wrench must fit between the pipe wall and the flange. In other words, the driver cannot be surrounded by so much mechanism that it cannot engage the nut it is used to tighten. It is also beneficial that the device be lightweight, easy to use, rugged enough to function in a dirty environment or in any horizontal or vertical orientation and it must move rapidly enough to run down the nut but still have controllable torque for final tightening.
Prior power torque wrenches use a variety of approaches to regulate the torque that is applied. One successful approach is to create torque by pulling a flat chain or other tension member from a drum or cylindrical surface that rotates around the driver. As used in this specification, the driver is that part of the wrench that grips the nut or valve head or other device to which torque is applied. A hydraulic or pneumatic piston-cylinder arrangement can pull the chain of such a wrench with a predetermined force so that the torque created by unwrapping the chain from the cylinder is established at the driver positioned at the center of rotation of the cylinder. When the force pulling on the chain is established at a certain level, when that amount of torque is applied to a nut or bolt it will tighten no further. One device of this nature is disclosed in U.S. Pat. No. 3,633,446.
In order to maintain the torque at the desired level, it is necessary that the chain be wrapped on the cylindrical surface in a single layer. If multiple layers are employed, the effective diameter of the cylinder or drum changes. It is also important that the cylinder from which the chain is pulled be small in diameter in order to conserve space around the driver. Accordingly, a power torque wrench driven by a flat chain pulled tangentially from a cylinder employs a small cylinder to conserve space and employs short strokes from the motor so that the cable pulled from the cylinder will cause it to rotate slightly less than 360.degree. per stroke. On such a device it is required to provide means to return the cylinder to its original position to re-wrap the chain around it.
Usually the cylindrical surface is mounted to an internal drive mechanism that employs a ratchet with a spring mechanism within the ratchet to provide the power to rewind the chain on the cylinder. Rewinding is done on what is called the return stroke. The use of the spring within the cylinder-ratchet assembly requires the use of space which enlarges the head. Also, since the head must be maintained as small as possible, a large spring cannot be used and the spring force from a small spring is limited. In addition, as the spring powers the return stroke, the rewinding force diminishes and as a result the return stroke is slow and frequently the spring does not completely rewind the device. With the limited force applied by a spring, particularly a small spring that will not overly enlarge the head of the torque wrench, the device becomes sensitive to dirt, lack of lubrication, and orientation with regard to vertical and horizontal. In addition the constant but unpredictable spring force will influence the torque provided by a wrench on its power stroke.
As used in this description, the term torque wrench includes devices that apply predetermined torque to rotating means or elements and it includes within its terms ordinary torque wrenches as well as screwdriving devices, devices to operate valves, door actuators and the like.
As used in this specification the term "motor" includes any power device that exerts force to pull the chain or tension element employed in this invention. Typically the motor will be a hydraulic or pneumatic piston-cylinder combination but the term is not limited to such devices.
The term chain or tension element includes any elongated flexible element that is capable of being wrapped around a cylinder and is able to resist a tension-producing force. Examples of tension elements are rope, cable, wire, chain and the like.
The term cylindrical surface, spool, drum and the like are used interchangeably in this specification to indicate a cylindrical element capable of rotating symmetrically around an elongated axis.
The term ratchet means includes any device that produces relative rotary motion between two elements in one direction that causes them to rotate together in the opposite direction. A typical ratchet-means is a ratchet.